The invention is related generally to rolls for treating webs of material and, more particularly, to a hydraulically supported roll having improved bending control.
In DE-OS 3640902, a hydraulically supported roll having bending control is disclosed in which the roll comprises a rotatable hollow cylinder, which forms the working roller circumference at its outer diameter, and a stationary crosshead extending lengthwise through the hollow cylinder to form a surrounding clearance space with the inner circumference of the hollow cylinder. The clearance space between the inner circumference of the hollow cylinder and the crosshead is completely filled, i.e., over 360.degree. viewed from the circumferential direction, with hydraulic fluid under a controllable pressure. Annular sealing elements arranged in a row along the crosshead are provided in the active plane. These sealing elements can be moved radially similar to a piston/cylinder unit until their circular sealing rim abuts the inner circumference of the hollow cylinder. The interior of the sealing element can be filled with hydraulic fluid. If the fluid in the interior of the sealing element is at a pressure that is higher than the pressure prevailing in the clearance space, then a positive force, which corresponds to the excess pressure, is exerted by the sealing elements against the inner circumference of the hollow cylinder. On the other hand, the interior of sealing element may be filled with hydraulic fluid at a pressure that is lower than the pressure prevailing in the clearance space. In this case, the pressure in the clearance space predominates and the zone located on the side opposite the sealing element, i.e., the zone 180.degree. circumferentially offset from the sealing element, exerts a positive force corresponding to the differential pressure against the inner circumference of the hollow cylinder. By suitably selecting the pressure in the sealing elements (when the pressure in the clearance space remains constant), the hollow cylinder, for example, can be pressed against a counterroll at one time and lifted from the counterroll, at another time, for instance, to enable a web to be fed into the roll nip. The construction of the sealing elements required for this type of operation is disclosed in DE-OS 3640902, which is a counterpart foreign application to U.S. application Ser. No. 07/126,773, filed Nov. 30, 1987, now U.S. Pat. No. 4,856,157 the disclosure of which is incorporated by reference herein.
When the hollow cylinder is pressed against a counterroll, a line pressure develops having a distribution dependent upon the pressures in the sealing elements. These sealing elements can be controlled individually or separately in groups. In one embodiment of a roll disclosed in DE-OS 3640902 and the above-mentioned application, the sealing elements, whose internal pressure can be lowered below that of the clearance space, are arranged on the side of the crosshead opposite the roll nip. Also, in order to modify the line pressure distribution on the side facing the roll nip, additional sealing elements, not at the same longitudinal position as the above-discussed sealing elements, are provided on the side of the crosshead facing the roll nip. The additional sealing elements can exert a positive force on the inner circumference of the hollow cylinder in the direction of the roll nip.
DE-AS 2230139 and EP 0201783 disclose rolls having sealing elements that can only exert a positive force against the inner circumference of the roll and in which the sealing elements are mounted in two rows offset by 180.degree. (viewed from the circumferential direction of the roller) such that the sealing elements mutually oppose each other in pairs.
The invention is particularly directed to a roll having an "internal stroke" in which the hollow cylinder is supported for radial movement, as a whole, relative to the crosshead. Such an arrangement is disclosed in DE-AS 2254392. The invention began with the realization of the problem that the dynamic effect of a sealing element whose internal pressure can be lowered below that of the clearance space is restricted by this internal pressure. If need be, the pressure in the sealing element can be lowered to zero. Then, on the opposite side of the cylinder, the pressure prevailing in the clearance space acts on the surface corresponding to the surface of the sealing element, i.e., the diametrically opposed surface. In many cases, however, it is desirable to exert a higher pressure at a specific roll location, for example, in order to influence the line pressure distribution at specific locations.